The invention relates to a method for the commutation of an electronically commutated, brushless, polyphase permanent magnet motor.
Polyphase permanent magnet motors nowadays are commutated increasingly electronically, i.e. brushlessly. The rotating rotary field in the stator winding with this is electronically produced, for example by way of a voltage/frequency converter. In order to be able to optimally convert the electrical power into mechanical motor power and in order to ensure a motor running which is as low vibration, smooth and low-noise as possible, it is necessary to synchronize the speed between the electronically produced circulating rotary field of the stator and the permanent magnet or magnets circulating therein. For this a position acquirement of the rotor is required which may either be effected by sensors provided on the stator side, for example Hall sensors but also by way of acquisition of the intrinsic induction arising in the stator windings. The first requires a measurement effort which is not inconsiderable, for the acquisition as well as for evaluation. The latter however in practice is only possible with the help of a separate measurement winding or however with block-commutated motors, since a measurement of the intrinsic induction without a great effort with regard to measurement technology is only possible in the phase without supply voltage when the voltage arising by way of intrinsic induction in the motor phase winding is not superimposed by the external supply voltage.
From U.S. Pat. No. 4,654,566 it is known, by acquiring the voltage induced within a winding, to determine the rotor position and to commutate the motor in dependence of the evaluated rotor position, i.e. specifically to determine the zero crossing of the voltage induced by the intrinsic induction in a motor winding and to carry out the reapplication of this motor winding to the supply voltage at a certain angle (commutating angle) from this point on. With this method the commutating point in time is matched to the current/present rotor position in that the commutating angle or the time interval determined by way of the dependence of the rotor speed is fixed from the zero crossing of the voltage induced in a motor winding. With this the commutation is always effected in dependence on the current position, however the commutation angle is always the same. An optimization of the commutating angle may however only be effected in dependence on the rotary speed so that the commutating method known from the above mentioned US patent can only be usefully applied when the motor is operated at a constant rotational speed.
Proceeding from this it is the object of the present invention to improve a commutating method of the specified type to the extent that with a low measuring effort an adaptation of the commutation angle may be effected in order to adapt the speed of the rotary field rotating in the stator to that of the rotor, and to ensure a low-vibration and smooth running of the motor in all rotational speed regions.
In one aspect, this invention comprises a method for commutating an electronically commutated brushless three phase permanent magnet motor (BLDC motor) in which the rotor position with respect to the stator and to the rotary field rotating therein is determined by way of the intrinsic induction in at least one motor winding, and the application of the external voltage to the motor winding is adapted, with respect to time, corresponding to the predetermined rotor position, wherein at least one motor winding the time from the point in time at which the winding current after switching off the voltage has essentially assumed the value zero until the zero crossing of the voltage induced in this winding is evaluated and in dependence on this time the external voltage, is again applied to this winding.
The method according to the invention is based on the principle that the commutation angle, i.e. the angle between the zero crossing of the voltage induced in a winding until reapplying to the same winding the supply voltage, is adapted in dependence on the deviation, between the rotary field rotating in the stator and the magnetic field circulating with the rotor, from the directed position. Since this adaptation is effected in every operating condition of the motor it is not only ensured for a certain rotational speed but for every possible speed. By way of this a higher efficiency and smoother running of the motor over the whole rotational speed range is ensured. Complicated measurements for acquiring the phase shifting as well as for computing the commutation angle are not required, only an acquisition of the time or of the angle from the zero crossing of the winding current up to the zero crossing of the voltage induced in this winding is required as well as a suitable control which, after a time interval dependent on this time or angle, preferably however of the same magnitude or after a preferably equal angle again dependent on this, effects the commutation, i.e. the reapplication of this winding to the supply voltage. This may be effected without further ado by way of the usually preconnected converter with a suitable control. The acquisition of the point in time when the winding phase current becomes zero as well as that of the zero crossing of the voltage induced in this winding have been known for a long time, the formation of a suitable control for commutating the motor in the above mentioned manner is clear to the man skilled in the art, which is why it is not gone into in detail here.
The method according to the invention thus with a relatively low expense with regard to measuring technology and control technology permits an adaptation of the commutation angle at each operating condition of the motor, in particular at each rotational speed. By way of this not only the efficiency but also the smoothness of running and noise emission of the motor is improved.
Preferably the commutation time is selected such that this is equal to the time in which computed from when the phase current becomes zero the zero crossing of the induced voltage is effected since then there is effected an excitation of the motor which is symmetric with respect to the zero crossing of the induced voltage, this favoring a smooth and low-noise running of the motor. It is however also conceivable that with special cases of application or motor designs a commutation time differing from this is selected, which however in any case is dependent on the previously evaluated time.
Advantageously the method according to the invention is applied in succession in each phase winding (this is to be understood as one or more motor windings which are allocated to the same phase), in order thus to ensure an exact as possible adaptation of the commutation angle to the actual motor condition.
Usefully the measurement of the intrinsic induction in the respective winding is effected only when this motor winding is separated from the voltage supply, in order to avoid a superposition of the external supply voltage and the internal induction voltage. The voltage then applied at the winding is caused exclusively by intrinsic induction and represents in the known manner the BEMF signal (back electromotive force).